Starch-containing biodegradable plastics have been widely commercialized in environmental applications because of their low prices. U.S. Pat. Nos. 4,016,117, 4,207,221 and 4,125,495, as well as British Patent 1,487,050, disclose the technologies of preparing starch-containing biodegradable plastics.
More specifically, U.S. Pat. Nos. 4,016,117 and 4,207,221 teach the addition of low molecular weight compounds, such as unsaturated fatty acids/fatty acid esters and unsaturated wax, to plastic-starch blends. The unsaturated double bonds in the additives can react with transition metal salts in soil and water to initiate auto-oxidation, thereby improving degradability of the plastic-starch blends. However, this approach has a major drawback, i.e., the low molecular weight additives worsens the mechanical properties of the blends.
U.S. Pat. No. 4,125,495 and British Patent 1,487,050, on the other hand, utilize a silane coupling agent or isocyanate as graft-modifier to the starch surface to improve the compatibility of starch and plastics. The mechanical properties of the plastic-starch blends is thus improved. However, modified starch is quite expensive. Furthermore, the biodegradation rates of such blend systems are rather low. Consequently, addition of low molecular weight unsaturated fatty acids/fatty acid esters or unsaturated wax to plastic-starch blends remains in large part the method of choice.
Whether the plastic portion of the starch-containing biodegradable plastics can really degrade is an unsettled issue. The plastic ingredient of conventional starch-containing degradable plastics is supposed to degrade via an auto-oxidation mechanism. However, as a matter of fact, the degradation is too slow to be observed. Some researchers have asserted that the plastic ingredient does not degrade by auto-oxidation and therefore the starch-containing biodegradable plastics are at best destructible, rather than degradable.
In the present invention, the degradability of a plastic is improved by adding thereto a reactive functional compatibilizer, instead of low molecular weight unsaturated fatty acids/fatty acid esters or unsaturated wax. As a result, the mechanical properties only slightly deteriorate, if at all.